Currently, known extendable/retractable screw-in implantable stimulation leads have an electrically active helix electrode. The helix electrode may be capable of extension and retraction from the header by being directly connected to the connector pin/distal coil subassembly. Turning of the connector pin results in the extension or retraction of the helix electrode from the header. In order for the helix to be extended or retracted, a thread/screw mechanism is required. The helix electrode is used as a threaded screw which turns against a thread post in the header. As the helix rotates, by turning of the connector pin, it engages the thread post, which in turn drives the helix into and out of the header.
Leads are also known which employ a fixed extended electrically active helix electrode and the invention is applicable to such leads as well.
Traditionally, the helix electrode of either fixed screw-in or extendable and retractable screw-in leads is made out of a round wire. The size of the wire should be small enough so as not to severely damage the heart tissue but thick enough to achieve good fixation while being visible under fluoroscopy. In addition, the outer diameter of the helix electrode should be sufficiently large to achieve a good and less traumatic fixation but not too small to core out the heart tissue. Currently, the appropriate range of the helix wire diameter is between about 10 mils (0.010 inches) and 13 mils (0.013 inches) and the outer diameter of the helix wire desirably ranges from about 0.040 inches to 0.070 inches. The outer diameter of the helix and the helix wire diameter must be balanced in order to create a first class helix electrode. In order to downsize the distal tip of a screw-in lead for a smaller lead, besides other design considerations at the distal tip, the helix wire size and outer diameter could be reduced to a certain extent. However, further reduction of the helix wire size or helix outer diameter would compromise the performance of the helix electrode in terms of electrode pacing threshold and helix visibility. The invention allows for more design options for the tip of a smaller lead without compromising helix performance.
Typical of the known prior art is U.S. Pat. No. 4,860,769 to Fogarty et al. which provides for an implantable defibrillator electrode including a flexible insulated guide terminating in a flexible distal portion which includes a conductive element and is of a predetermined configuration, such that it may be extended to a linear configuration by the application of a concentric or axial force and upon the termination of such force assumes the predetermined configuration. The conductive portion of the electrode may be a spiral of flat wire helically wound on a generally cylindrical, non-conductive stem.
Another example of the prior art can be found in U.S. Pat. No. 4,920,979 to Bullara which discloses a circumneural electrode assembly including a supportive flexible and insulating matrix formed into two oppositely directed helical portions which are centrally joined, and have free outer ends. The helical portions extend circumferentially at least one full turn, and preferably about one-half additional turn, for a total extent in the range of 360 degrees to 720 degrees. A thin and flexible conductive ribbon, preferably of surface-roughened platinum, is secured to the inner surface of one of the helical portions, and multiple electrodes can be provided on one or both portions. A connecting wire or cable extends from the electrode and matrix for coupling to an electronic package which is normally implanted elsewhere in the patient's body.
Still another example of the prior art can be found in U.S. Pat. No. 5,964,702 to Grill, Jr. et al. which discloses a helical nerve cuff electrode which is provided for encircling a nerve trunk or other body tissue with at least one medication or electrically energy conductive member disposed along the length of the helical cuff. The cuff includes a self-curling sheet of non-conductive material laminations which are collectively self-biased to curl into a tight helix.